This invention relates generally to electrophotographic printing, and more particularly, it is directed to a developer auger which is used in a development system of an electrophotographic printing machine.
The features of the present invention may be used in the printing arts and, more particularly, in electrophotographic imaging machines. In the process of electrophotographic image formation, a photoconductive surface is charged to a substantially uniform potential. The photoconductive surface is selectively exposed to record an electrostatic latent image corresponding to the informational areas of an original document being reproduced. Thereafter, a developer is transported into contact with the electrostatic latent image. Generally, the developer consists of toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules of the developer onto the latent image. The resultant toner particle image is then transferred from the photoconductive surface to a final substrate such as plain paper and permanently affixed thereto.
Vagaries induced into the xerographic process by excursions in temperature and relative humidity (RH) create well known problems. While most high speed xerographic electronic printers operate in relatively well controlled environments, many printers and copier /duplicators do not. Even printers designed for a relatively benign computer room environment (4850.TM. for example) are required (specified) to work reliably over an environmental range between 80.degree. F./80% RH and 60.degree. F./10%.
A change in temperature can alter the sensitivity of a photoreceptor's Photoinduced Discharge Curve (PIDC). More exposure energy is required to discharge a cold photoreceptor compared to the same photoreceptor at a higher temperature. Consequently, less exposure is needed if a cold photoreceptor is warmed to a higher temperature by an electrical blanket heater (U.S. Pat. No. 3,887,367).
Temperature also plays a role in establishing the relative humidity within the developer sump. Relative humidity in turn influences the developer's triboelectric charging properties. For example, when the relative humidity is high, toner charge to mass ratio (tribo) tends to be low, and conversely, high when the RH is low. Because the triboelectric charge on a toner particle tends become smaller as the toner concentration increases, toner concentration (TC) can be adjusted to compensate for humidity changes that might otherwise de-stabilize development Unfortunately, there are situations where this approach does not work.
Consider for example, a printer that has been operating in a warm, low RH environment (high TC) which is then is shut down and allowed to equilibrate in a cool, high RH, ambient environment. Because the TC is high, and the prevailing RH at start up is high, the resulting developer tribo charge will be low and tend to produce prints with over developed solid areas and high spurious background.
Normally, as the machine continues to run the problem will correct itself because the xerographic cavity warms up causing the relative humidity to fall, and/or the control system permits the toner concentration to run down to an appropriate tribo level. But, until the new tribo equilibrium is attained, print quality is likely to be unacceptable. Therefore, relying on normal toner consumption to lower the TC as a means to adjust tribo may not always be an appropriate strategy for machines which have been idle for an extended period of time.
One countermeasure that has been proposed in the past to stabilize the xerographic process against operating extremes in temperature and humidity is to employ an external, electrically heated, developer housing blanket to warm the developer housing above ambient room temperature. The heating blanket elements can be activated or de-activated in response to a control circuit that monitors the development housing's temperature and/or humidity. This can be done whether the machine is running, or in standby (if the control circuits are powered). The external heating blanket can be deactivated when the heat generated during normal operation of the development housing warms the developer to, or above, some predetermined temperature.
A disadvantage of the external heating shroud is that because it heats from the outside inward, developer in the vicinity of the outer walls will be at a higher temperature than developer on the interior of the housing. Moreover, because the heating shroud is the warmest part of the housing, unless it is thermally insulated on the outside, a large portion of the heat it generates will be lost to the outside environment. A typical high speed printer or duplicator developer housing converts 50 to 90 watts of mechanical power into heat during normal operation. Without forced air cooling, the temperature inside the housing can easily rise to as much as 25.degree. C. above ambient. In this case, insulation on the outside of the heating shroud acts as a thermal barrier and will impede the rate at which the housing can rid itself of excess heat.